The invention relates to a hydrodynamic bearing.
FIG. 3 shows an example of a hydrodynamic bearing which is known in the art. The hydrodynamic bearing comprises: a shaft 30 having two radial flanges 34 and 35 which are axially separated from each other by a predetermined distance; a sleeve 31 through which the shaft 30 is passed and which is pressingly held between the radial flanges 34 and 35; a radial dynamic pressure bearing portion 37 which is formed between the shaft 30 and the inner peripheral face of the sleeve 31; axial dynamic pressure bearing portions 38 and 39 which are formed between the end faces of the sleeve 31 and the radial flanges 34 and 35, respectively; and pressure equalization passages 36 which are respectively formed in the sleeve 31 and at plural positions on a circumference and radially outside the axial dynamic pressure bearing portions 38 and 39, and through which the end faces of the sleeve communicate with each other. When the shaft 30 relatively rotates with respect to the sleeve 31, a dynamic pressure is generated in the radial dynamic pressure bearing portion 37 and the axial dynamic pressure bearing portions 38 and 39. The sleeve 31 is integrally formed by pressingly fitting a cylindrical inner sleeve 32 into a cylindrical outer sleeve 33. As shown in FIG. 4, cutaway portions 42 through which the end faces of the sleeve communicate with each other are formed in the outer peripheral face of the inner sleeve 32 and at diametrically opposite positions, respectively. When the inner sleeve 32 is pressingly fitted into the inner peripheral face of the outer sleeve 33, the cutaway portions 42 are formed as the pressure equalization passages 36. The inner peripheral faces 33a and 33b of the end portions of the outer sleeve 33 are disposed so as to be in close proximity to and opposed to the side faces 34a and 35a of the radial flanges, and also to surround the side faces 34a and 35a, respectively. Furthermore, liquid sealing portions 40 and 41 due to the surface tension are formed by gaps between the flange side faces 34a and 35a and the inner peripheral faces 33a and 33b of the outer sleeve, whereby the liquid in the bearing is sealed. Such a hydrodynamic bearing of the prior art is disclosed in, for example, the specification of the U.S. Pat. No. 4,795,275.
In the hydrodynamic bearing of the prior art, during a relative rotation of the shaft 30 and the sleeve 31, a dynamic pressure acts on the dynamic pressure bearing portions 37, 38, and 39. When the pressure balance in the bearing is lost, the liquid in the bearing may leak from the sealing gap 40 and 41 between the flange side faces 34a and 35a and the sleeve inner faces 33a and 33b. In order to maintain the pressure balance, the pressure equalization passages 36 are formed so that the liquid is supplied into the pressure equalization passages 36, thereby preventing the liquid from leaking.
In the hydrodynamic bearing of the prior art, the bearing consists of the three parts, i.e., the shaft 30, the inner sleeve 32, and the outer sleeve 33, and hence high coaxiality among the parts is required, so that it is difficult to work and assemble these parts. Because of this, these parts must be accurately worked, thereby increasing the production cost. Since the inner peripheral face of the outer sleeve 33 is formed by a cylindrical face, the leakage of the liquid inevitably occurs in spite of the existence of the pressure equalization passage 36.